Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/%25D0%25B0%25D0%25BB%25D0%25B0%25D0%25B2%25D0%25B5%25D1%2580%25D0%25B4%25D1%258B' do
    "Hello World"
  end
end